A thermal fuse device, a current fuse device, a polymer PTC device, or the like is used as a protection device which interrupts an excessive current when the excessive current flows during charging or discharging of a cylindrical lithium-ion secondary battery. In particular, the polymer PTC device is useful in that it can be positioned by incorporating into a sealing plate of the secondary battery, a battery pack which is particularly composed of many of the secondary batteries, to be compact. However, a commercially available PTC component is disadvantageous compared to a fuse device consisting of a metal when a large amount of current is passed through the PTC component because the PTC component has a larger resistance than the metal.
In view of the above described problem, it was proposed, for example, that a spacer is used in place of the PTC device inside the sealing plate in the cylindrical lithium-ion secondary battery cell (see the Matsushita Technical Journal reference referred to below). However, when the spacer is used, there is a problem that a protection from the excessive current cannot be ensured.
The fuse device as a protection device used in a secondary battery is advantageous in that it has a low resistance and can pass a large amount of current. However, since the fuse device is not directly disposed in a cell inside but is disposed on a side surface of the battery or a connecting portion between the batteries, there is a disadvantage that a battery pack becomes larger. In addition, when an excessive current is passed through a circuit, the fuse device protects the circuit by blowing (fusing) to interrupt the excessive current, while there is a disadvantage that once the fuse device blows, the cell cannot be used.
Matsushita Technical Journal, Vol. 52, No. 4, August 2006, pp. 31-35, is a prior reference.